Embodiments of the invention relate generally to structural framing systems and, more particularly, to structural connection mechanisms that are included in a structural framing system (primary, secondary, or other), to provide and allow for discontinuous elastic behavior of such system(s) for load conditions where plastic ductile behavior of commonly used beams and connection mechanisms would otherwise be relied upon. The discontinuous elastic behavior as described herein is achieved by constructing one or more of the structural framing system's connections, e.g., a beam to column connection, in a manner that comprises a zone in the load path of the connection where the stress strain behavior is more elastic than the elastic modulus of the base materials from which the connection is constructed of if constructed as a rigid connection, would predict.
In areas prone to seismic loading events, structures such as buildings and bridges often include seismic force resistive systems integrated therein. These seismic force resistive systems attempt to protect the structure and control damage, loss of life and contents by mitigating the detrimental effects of forces associated with such loading events, such as by safely enhancing ductility and damping characteristics of the structure. One response of a structure to a seismic event is drift of the structure, i.e., lateral deflection. The amount of drift experienced by a structure for a given seismic load is, in part, determined by the stiffness of the structure, with drift being smaller for stiffer structures and larger for less stiff structures of equal mass.
The amount of drift allowed for a structure (i.e., drift limits) is prescribed by building codes and is dependent upon many things, generally including the type of forces imposing the drift, such as: seismic, wind, or other transient loads; building construction; use of the structure; finishes attached to or contained inside the structure; etc. Probability of occurrence, expected magnitude of load, and occupancy are given strong consideration as well. In structures where relatively large amounts of drift resulting from seismic events are deemed by building codes to be acceptable, it is often desirable to take maximum advantage of such allowance so as to reduce the required strength of the structure as compared to stiffer structures designed for less drift. A similar approach may be desirable relative to other transient loading conditions which result in horizontal or vertical deflections of structures. In the case of seismic loading, current codes allow drift that in many circumstances results in flexure of conventional rigid connections that exceeds the elastic limits of materials used to construct such conventional connections. To insure safety, the conventional method of achieving allowable drift for seismic loads therefore relies on the plastic ductile behavior of a designated portion of the structural frame whereat the strain will exceed the elastic limits, in a manner which is safe but predicted to cause damage to structural components.
Considerable prevalent existing technology achieves the allowable amount of drift resulting from seismic events through a “weak beam-strong column” philosophy where beam components in the structure exhibit elastic behavior at low levels of seismic loading, followed by inelastic plastic ductile behavior as seismic loading increases within the service load range, at a prescribed location near columns of beams with end moment resistance or at the intersection of a beam and chevron cross brace between floor diaphragms. In a weak beam-strong column structure, the columns are expected to perform in an elastic manner. In the weak beam-strong column philosophy, it is presumed that seismic events that load structural components, most notably beams and braces, beyond their elastic capacity, i.e. plastic inelastic behavior, will result in a structure that is misaligned, exhibiting deformed structural members following such an event. It is further presumed that such misalignment and deformation may be significant enough to render the building uninhabitable and in some cases unrepairable. That is, the plastic inelastic behavior of beams experienced at higher seismic loading may result in a condition where repair is not practical or economical following seismic events.
Another drawback to existing weak beam-strong column technology is that construction of systems employing the philosophy typically require field welding the connection of beams to columns, or of beams to beam stubs in the case of columns shop fabricated as “trees”. Beams with large (thick) flanges are often required by the structural design. Compared to shop fabrication, the limitations of field welding of these large flanges (e.g., shortcomings of SMAW or FCAW processes, lack of heat treatment, more variable environmental conditions, etc.) leaves welds susceptible to flaws which can be controlled to a higher degree in a shop environment where: a) additional more advantageous welding processes may be employed; b) pre- and post-weld heat treatments may be used; c) environmental conditions that effect weld quality can be controlled; and d) positioning techniques employed. For example, the field welding of such flanges often occurs in outdoor job site environments at remote and elevated areas, with completing of the weld possibly taking several hours or more in damp, windy, cold conditions, such that moisture, pre-weld temperature of the weldment, interpass temperature, and ambient humidity may adversely affect consumables and the strength of the weld. Post-weld heat treatment of structural connections in the field, i.e., construction sites, is impractical and seldom performed, leaving no thorough remedy to residual stress in and around the weld zone induced by heating and cooling from the field welding process. Flaws most often associated with environmental conditions, access, and less than optimal positioning of the weld joint may also occur more frequently when field welding the beams to columns compared to shop welding, such as slag inclusions and lack of fusion which is sometimes found near the web in the lower flange of the beam where mechanized wire brushing and grinding is not feasible, visibility of the weld puddle is impaired, and inadequate overlapping of starts and stops tend to occur.
Another prior art mechanism for achieving ductile behavior, a variation of the strong beam-weak column approach, includes “fused” connections that take advantage of friction or plastic deformation or sacrificial components to provide the flexure necessary for achieving the allowable drift for seismic loading. That is, fused connections are intended to undergo plastic deformation, and/or friction modulated movement along faying surfaces or other (non-elastic) failure during a seismic event, allowing some form of permanent displacement or deformation of the connection as a result of a design level seismic event. While such fused connections may purport to have self-realignment properties, such self-realignment associated with the fused connections relies in fact primarily on the response of bracing and columns, and is achieved without the assistance of the connection.
Therefore, it would be desirable to provide a structure and associated seismic force resistive system that extends the elastic range of structural framing elements such that the allowable drift, more specifically the inter-story drift, i.e., incremental drift between adjacent floor levels, for a design seismic event, may be achieved without plastic deformation of beams, columns, or bracing. It would further be desirable for such seismic systems to enable self-realignment of the structure to its pre-seismic event orientation and save economical realignment in instances where design loads and allowable drift have been experienced or narrowly to moderately exceeded. It would be still further desirable for such seismic systems to make economical use of fabrication advantages typically associated with controlled factory assembly and fabrication for critical heavy welds as compared to field assembly such as positioning, gas metal arc welding, submerged arc welding, normalizing, heat treatment and stress relief, and provide for greater use of bolted field connections in lieu of field welded connections, thereby reducing field man hours per connection and quality control requirements relative to field welded connections vs. shop welded connections.